Methods of binding pigment particles to phosphor particles are described in the literature. These materials are mostly used in cathode ray tubes where the pigment acts as a filter to improve the color purity and contrast of the emitted light. Both organic and inorganic pigment binders are used in industry. In U.S. Pat. No. 3,875,449, issued Apr. 1, 1975 phosphor particles are surrounded by a continuous coating of such a material, and the coated particles are then encapsulated in a protective resin coating, for example polyvinyl alcohol. In U.S. Pat. No. 3,886,394, issued May 27, 1975, phosphor particles are covered with a filter material adhered to the surface of the particles by a gelatin layer, for example, polyvinyl pyrrolidone. The filter particles are thus embedded or dispersed in the adherent gelatin layer. In U.S. Pat. Nos. 4,020,231, and 4,128,674, issued Apr. 26, 1977, and Dec. 5, 1978, respectively, and assigned to the present assignee, a light modifying layer is formed on the phosphor by coprecipitation of silica and pigment particles. In U.S. Pat. No. 4,021,588, issued May 3, 1977, a filter material is precipitated directly onto the phosphor particles and the particles are then sintered to adhere filter particles to the surfaces thereof. U.S. Pat. No. 4,049,845 issued Sept. 20, 1977, describes various organic binder systems that may be used. These are usually applied as lattices.
With the exceptions of U.S. Pat. Nos. 4,020,231 and 4,021,588, the above patents all describe organic-based coating systems. The major disadvantage of these systems is that the phosphor particles must retain their organic coating in order to retain the light modifying particles. Thus, the strong oxidizing agents and/or heat normally encountered during phosphor reclaim operations (in which a phosphor is reclaimed from the face-plate of a cathode ray tube) must be avoided. This requirement essentially renders such reclaim operations impractical, since organic-based systems are also used to adhere the coated phosphor particles to the faceplate. In practice then, pigment or other light modifying particles are then removed during the reclaim operation and the reclaimed phosphor particles are then recoated prior to being redeposited on a faceplate.
Another disadvantage of organic-based coatings is that they tend to render coated particles unwettable by water based slurry systems used to form cathode ray tube screens. Such poor wettability leads to poor dispersion of the phosphor particles and consequent poor screen quality.
Other coating techniques which avoid the use of organic materials have proved unsatisfactory for other reasons. For example, the coprecipitated coating of U.S. Pat. Nos. 4,020,231 and 4,128,674 tend to have poor adherence when subjected to the handling associated with screening operations. The sintered coating of U.S. Pat. No. 4,021,588, while exhibiting good adherence, requires relatively high temperatures (at least about 800.degree. C.) for its formation.
U.S. Pat. No. 4,473,634 describes an inorganic binder system which uses water glass (potassium silicate) as the binder. However, the drawbacks in this process are that the waterglass binder is slurried with the phosphor and pigment and then dried into a hard mass which must be ground back into a fine powder. The grinding process is labor intensive and results in poor material efficiency due to large amounts of undersize particles resulting from the grinding process.